College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]
CHAPTER9. APPLICATIONS OF HEAT TRANSFER-HEAT EXCHANGERS Heat exchangers are classified according to flow arrangement and type of construction. The simplest type
is the one that the hot and cold fluid flow moves in the opposite and the same direction of the fluid flow.
The cross-fluid flow heat exchanger is the fluid flow moves perpendicular into the fluid. The counter fluid
flow heat exchanger is classified according to the direction of the fluid flow, the fluid flow moves in the
opposite direction, however the parallel (shell tube) fluid flow arrangement is defined as the fluid flow
moves in the same direction, as shown in Fig. 1.3. The compact heat exchanger is a very dense arrays of
finned tubes which is used for liquid and gas.
a. b.
Fig1.3. a) Parallel shell tube heat exchanger, b. Counter shell tube heat exchanger
Fig.1.4. Cross-fluid flow heat exchanger arrangement
College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]
a.
b.
c. d.
Fig.1.5. a. Counter heat exchanger, b. U-tube heat exchanger, mixed Flow heat exchanger, and d.
compact heat exchanger.
Overall heat transfer Coefficient
The overall heat transfer of the finned heat exchanger can be estimated as:
1
UA=
1
UAc=
1
UhAh=
1
ηohcAc+
1
ηohhAh+ Rw +
Rfc
ηoAc+
Rfh
ηoAh
Where ηois the overall surface efficieny = 1 −Afin
A(1 − ηf)
Rfcand Rfhare the fouing resistance in the cold and hot side, respectively.
The total heat transfer rate is
q = Ahηo(Tb − T∞)
Where the single fin efficiency can be estimated as:
ηfin =tanh(ML)
ML, M = √
2h
kth where th is the thickness
College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]
Table 1.1. Fouling Resistance
Table 1.2. Values of the overall heat Transfer Coefficient
For unfinned tubular heat exchangers, the overall heat transfer coefficient is
1
UA=
1
UAc=
1
UhAh=
1
hcAc+
1
hhAh+
ln (DoDi
)
2πkL+
Rfc
Ac+
Rfh
Ah
Parallel Flow Heat Exchanger
The two fluids move in the same direction. The temperature distribution associated with the
parallel flow heat exchanger initially large and decays with increasing x.
Fig.1.1. Parallel flow distribution temperature
College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]
The total heat transfer can be estimated as
qc = mc cpc(Tco − Tci), qh = mh cph(Thi − Tho)
The total heat can be evaluated based on the log mean temperature as:
q = UA∆Tlm where ∆Tlm =∆T2 − ∆T1
ln (∆T2∆T1
)
Counter Flow Heat Exchanger The two fluids move in opposite direction.
Fig.1.2. Counter flow distribution temperature
The hot and cold heat capacity rate can be represented by:
Ch = mcph, and Cc = mc_pc
College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]
Special heat exchanger conditions:
Fig1.3. Heat capacity rate for different conditions
Effectiveness of Heat Exchanger
The ratio of the actual heat transfer rate to the maximum possible heat transfer rate.
qmax = Cmin(Thi − Tci)
Cmin is either equal to Ch or Cc whichever is smaller.
ε =q
qmax=
Ch(Thi − Tho)
Cmin(Thi − Tci)
The number of transfer unit (NTU) is an important parameter in a heat exchanger design.
ε = f (NTU,Cmin
Cmax)
NTU =UA
Cmin
Table 1.3 shows various formula of NTU which relies on the configurations of the heat
exchanger.
The capacity rate ratio is
Cr =Cmin
Cmax
College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]
Table 1.3. Heat exchanger effectiveness definitions
Log Mean Temperature Difference Method for Multi-pass and Cross-Flow Heat
Exchanger
The pervious equation can be used to calculate the Multi-pass and Cross flow heat exchanger by
adding the correction factor.
∆Tlm = F∆Tlm,CF
The single tier cross flow heat exchanger correction factor can be evaluated from Fig. 1.1.
The multiple-pass shell and tube heat exchanger correction factor can be evaluated from Fig.1.2.
The single pass unmixed heat exchanger correction factor can be evaluated from Fig.1.3.
The multiple-pass one fluid mixed and the other unmixed heat exchanger correction factor can be
evaluated from Fig.1.3.
College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]
Fig.1.1. Correction Factor of Tube –Shell heat Exchanger (two tubes)
Fig.1.2. Correction factor of Multiple-pass (Four tubes) shell and tube heat exchanger
College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]
Fig.1.3. Correction Factor of a single pass of unmixed flow heat exchanger
Fig.1.4. Correction Factor of one fluid mixed and other fluid unmixed
College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]
College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]
College of Engineering Summer Session- 2015 Heat Transfer - ME 372 Dr. Saeed J. Almalowi, [email protected]